#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int N, M;

bool C(ll x, int mid, vector<ll>& v){
  ll m = x + v[mid];
  int ret = 0;
  vector<ll> w;
  rep(i,v.size()){
    if(i==mid) continue;
    w.push_back(v[i]);
  }
  sort(ALLOF(w));
  int a=0, b=w.size()-1;
  while(a<b){
    while(a<b && w[a]+w[b]<=m) a++;
    if(a<b && w[a]+w[b]>m){
      ret++;
      a++;
      b--;
    }
  }
  return ret < M;
}

int main(){
  cin >> N >> M;
  vector<ll> v;
  ll x;
  cin >> x;
  rep(i,N-1){
    ll a;
    cin >> a;
    v.push_back(a);
  }
  if(N==2){
    cout << v[0] << endl;
    return 0;
  }
  sort(ALLOF(v));
  if(!C(x, 0, v)){
    cout << -1 << endl;
    return 0;
  }

  ll ret = v[0];
  int lb = 0, ub = v.size();
  while(ub-lb>1){
    int mid = (lb+ub)/2;
    if(!C(x, mid, v)) lb = mid;
    else ub = mid;
  }
  ret = min(ret, v[lb]);
  cout << ret << endl;
  return 0;
}